Rhenium thin film resistors



y 7, 1968 c. FELDMAN 3,382,100

RHENIUM THIN FILM RESISTORS Filed Sept. 14, 1965 INVENTOR CHARLES PELDMAM ATTORNEY United States Patent 0 3,382,1ii0 RHENIUM THIN FILM RESKSTORS Charles Feldman, Alexandria, Va., assignor to Melpar,

Inc., Falls Church, Va., a corporation of Delaware Continuation-in-part of application Ser. No. 173,261,

Feb. 14, 1962. This application Sept. 14, 1965, Ser.

13 Claims. (Cl. 117-217) The present invention relates generally to thin film resistor elements and methods for manufacturing such elements, and more particularly to an annealed rhenium thin film resistor wherein annealing is accomplished during and after deposition of the rhenium metal on a sub strate.

This is a continuation-in-part of my copending application, Rhenium Thin Film Resistors, Ser. No. 173,- 261, now abandoned, filed Feb. 14, 1962.

In the past several years, miorominiature circuit elements have been employed in electronic circuitry because of their small volume and weight. This minuteness in size and weight is particularly advantageous in aircraft, rockets, spacecraft, etc., because of the extreme cost necessary to launch every pound of equipment. In microminiature elements, resistors are formed by depositing, in a vacuum, a thin film of resistive metal on an insulating amorphous or uncrystallized substrate. Thin film resistors have, in the past, generally not proven satisfactory since their electrical characteristics are unstable and subject to change with time elapse and temperature variation caused by environment and electrical power dissipation.

The stability of a vacuum deposited thin film on an amorphous substrate depends chiefly on the melting point of the metal, and its density. These factors are related to the bonding forces of the metal atoms to the substrate and to the atoms themselves. In general, the higher the melting point of the metal, the more stable the film be cause the atoms and molecules have greater binding forces between themselves. Tungsten, for example, forms very stable resistive films in a high vacuum, however, it deteriorates when exposed to air or water vapor. Carbon is usually undesirable as a resistor because it has a very high resistivity and a large temperature coefiicient of resistance, rendering it an unstable element in varying environments. Tantalum, which has a melting point of about 2990 C., is not sufficiently stable to be employed as a thin film resistive element for microminiature purposes. The refractory element between tungsten and tantalum is rhenium, a comparatively unexplored metal with properties that lead to films having very stable electrical characteristics. The vapor pressure of rhenium, -at its melting point, is higher than tantalum thus making rhenium easier to evaporate but of sufiicient electrical stability for utilization in thin film devices.

I have found that rhenium has highly stable electrical characteristics if it is deposited on a substrate maintained at an elevated temperature. The elevated temperature of the substrate is maintained subsequent to complete deposition of the metal for a period varying from 30 to 90 minutes so that the metal is annealed. Annealing improves the electrical characteristics of the rhenium film resistor by improving the crystal structure to fix the film thereon, thereby preventing most relative movement between the crystal substrate and the metal during aging and temperature variation. Also, annealing causes the film to become slightly oxidized so that very small changes of the film occur when it is exposed to air. The annealing process employed during and after film deposition considerably aids the mechanical strength of the substrate and the thin film formed thereon since stresses which may be set up in the substrate during deposition are relieved. Relief of the stresses and strains is advantageous because "ice instability caused by relative motion between the substrate and film relative variation during the aging process is greatly reduced.

It has been found that the temperature coefficient of resistance for different resistance values of the film may be controlled by varying the temperature of the substrate during deposition, For a highly resistive film, i.e., one of small width, a zero temperature coefficient of resistance occurs when the substrate is maintained at higher temperatures than for low resistivity films.

To deposit rhenium on a substrate, an electron beam must be focused in a vacuum on a block of the metal to effect vaporization. Other types of heating cannot generally be employed because of the high melting point of rhenium, 3180 C. The vaporized metal propagates through the vacuum to the heated substrate surface to form a film of desired thickness. After the film and substrate are removed, a coating of dielectric, preferably silicon dioxide, is applied to prevent further oxidation of the rhenium when it is subjected to the atmosphere. A high degree of oxidation, of course, is deleterious to a rhenium thin film resistor since it results in poor contact between the film and the substrate, thus adversely affecting film stability.

It is an object of the present invention to provide a thin film resistor having stable electrical characteristics for wide temperature values and long time periods.

Another object of the present invention is to provide a method of manufacturing a rhenium thin film resistor having stable operating conditions.

A further object of the present invention is to provide a rhenium thin film resistor wherein the temperature c0- cfi'lcient of resistance of the film is accurately controlled at the time of manufacture.

Another object of the present invention is to provide a rhenium thin film resistor and the method of manufacturing same, wherein the resistor does not deteriorate when it is exposed to air or water, has reasonable resistance values and does not materially change its characteristics as temperature conditions vary in the environment in which it is located.

Still another object of the present invention is to provide a new and improved method for manufacturing a thin film resistor wherein the temperature coefficient of the film is accurately controlled by maintaining the substrate at an elevated temperature during deposition of the metal vapors thereon.

It is still another object of the present invention to provide a new and improved thin film resistor manufactured from rhenium, which resistor is annealed to prevent stresses between the substrate and film, and to control the resistivity of the film.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a schematic diagram of the apparatus employed for deposition of a thin film on a substrate; and

FIGURE 2 is a cross-sectional view of a thin film resistor formed by the process ofFIGURE 1.

Reference is now made to FIGURE 1 of the drawings which discloses the apparatus employed in carrying out the method of the present invention. An electron beam gun 12 and a cubic block of rhenium 13 are positioned within an evacuated vessel 11. Energization of electron beam gun 12 is established by the high voltage power supply 24, connected through suitable terminals in the exterior of container 11. Electrons are emitted by gun 12 and focused on a very small area of cubic block 13, having a side of approximately /2", so that a puddle is formed in the center of the side exposed to the electron beam. The rhenium block is supported by a pair of tungsten legs 14, tungsten being employed because of its lack of heat conductivity. Shutter rotates between the electron beam gun and block 13 to intermittently interrupt the electron beam impinging on the block. Positioned directly above the shutter when it is in a plane above block 13 is a mica mask 16, containing apertures 17 commensurate with the pattern to be formed on glass substrate 18. Substrate 18, being located above the mask 16, is any type of electrical insulator or dielectric generally employed for micro-circuitry element but it is preferably formed from a silica glass, ceramic or noncrystal material.

Located immediately above the substrate 18 is a quartz lamp heater 19 which maintains the substrate at temperatures ranging between 400 and 700 F. both during the deposition process and for 30 to 90 minutes thereafter. Opposite ends of heater 19 are connected via wires 21 and 22 to a suitable low voltage power supply to cause radiant heating from element 19.

It is to be understood that each of the elements 15, 16, 17, 18 and 19 within vacuum vessel 11 is maintained in place by suitable tungsten rods. The necessary vacuum, 10 mm. of mercury, is maintained within chamber 11 by a vacuum pump connected to pipe 23.

Prior to operation of electron beam gun 12, the entire chamber is subjected to a vacuum by means of pipe 23 and the vacuum system connected thereto. During this initial vacuum operation, the substrate 18 is maintained at 500 C. When the deposition process is initiated, the temperature of heated substrate 18 is controlled by the amount of heat emanating from heater 19.

In operation, the electron beam from gun 21 forms a puddle on the surface of block 13 which is being bombarded. The beam is focused sufficiently to cause the metal to be melted and vaporized into the vacuum atmosphere of chamber 11. The vaporized particles pass through the aperture 17 and mask 16 and impinge on the surface of substrate 18 facing block 13. The pattern formed on the glass substrate 18 is controlled by the shape of the aperture 17 of the mask 16.

To monitor the thickness of the deposit formed on substrate 18, light source 25 and photocell 26 are provided. Light from source 25 is propagated through port window 27 to the surface of substrate 18 and is reflected therefrom through port hole 28 to photocell 26. When the deposit is of the required thickness, light impinges on the cathode of photocell 26 and a signal is generated to terminate the electron beam emanating from gun 12.

Despite the vacuum maintained within chamber 11, a slight degree of oxidation results as the metal particles impinge on substrate 18 due to the heat applied to the substrate by element 19. This is advantageous because the long term stability of the electrical characteristics of the rhenium thin film resistor are enhanced by a very slight degree of oxidation between the film and the substrate.

By controlling the amount of heating during the deposition process, the temperature coefficient of resistance of the film may be accurately controlled for difierent resistance values. For example, films of rhenium deposited at 500 C. have a zero coefficient at about 50 ohms/square while those deposited at 600 C. have a zero coefiicient at about 350 ohms/square. The terms ohms/square is one being presently employed in the art for determining the resistivity of a thin film element. This term refers to the resistance of an element when a lead is connected across one complete edge of the element and a second lead is connected across the entire opposite edge of the element. These leads are connected at all points to the edge with which they are associated, thus resulting in an ohmic value of resistance between the two edges for any size square. It is thus seen that by varying the temperature of substrate 18 during the deposition process, the temperature coeificient of resistance may be accurately 4 controlled, thereby increasing stability of the film for temperature variations in use.

A high temperature limit of the range during the deposition and anneal steps depends to a great extent upon the particular substrate material. A quartz substrate, for example, allows a 700 C. limit, while other substrate materials require lower temperatures. In general, then, the approximate temperature of 700 C. should not be exceeded since above that critical temperature the film Will chemically react with the substrate. The temperature coetficient will also change when reaction occurs.

A number of reasons exist for annealing and depositing at above 400 C. Depositing at lower temperatures gives a higher temperature coeficient of resistance. One would like as low a temperature coefficient as possible. As an example, a SOOQ/square resistor made at 200 C. gave a temperature coefiicient of 265 ppm, while the usual temperature coetficient of the same value resistor deposited at 500 C. is approximately 95 p.p.m., almost a 300 percent difference between temperature coefiicients. In addition, it is evident that a resistor made and annealed at low temperature can never be taken to a higher temperature without change. Thus a film deposited at 200 C. can never be carried to 300 C. without suffering a change of characteristics. The stability with time or age is also improved by a high temperature annealing I have found that films formed at low temperatures generally deteriorated more rapidly than those made in the temperature range indicated. Adherence of the film is also better at high temperatures.

Subsequent to deposition of the rhenium film on substrate 18, the heater 19 remaines at the same temperature as it was during deposition for a period of between 30 to minutes. This results in aging of the film, and annealing of the film and the substrate to relieve any stresses that may be formed during the deposition process. Also it has been found that by maintaining the substrate at an elevated temperature subsequent to deposition, the long term stability of the substrate is maintained.

After the 30 to 90 minute period has elapsed, the substrate is allowed to cool and is then removed from chamber 11. A protective covering layer of silicon dioxide is then applied to the rhenium to prevent further oxidation thereof in the atmosphere. Such oxidation, if permitted to occur, would result in deterioration of the formed thin film during aging. This of course would result in electrical characteristics which are deleterious.

Reference is now made to FIGURE 2 of the drawings which discloses the silica glass substrate 18 upon which is formed the annealed rhenium thin resistor film 29 and the silicon dioxide protective layer 31. The metal film element 29 may be of the same or different widths depending upon the desired circuit component fabricated. Also the values of the resistances may be altered by varying the surface area of the deposited film on the substrate.

While I have described and illustrated one specific embodiment of my invention, it will be clear that variations of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

I claim:

1. The process of making highly stable thin film resistors which comprises, in a vacuum environment, the steps of depositing vaporized rhenium in a thin film on an electrically insulating substrate heated to a temperature within the range from 400 C. to 700 C., and thereafter annealing said film at a temperature within said range for a period of time sutficient to stabilize the electrical and physical characteristics of the thin film resistor.

2. The process of claim 1 wherein said substrate is a ceramic material.

3. The process of claim 1 wherein said substrate is a silica-bearing glass.

4. The process of claim 1 wherein said period of time is from 30 to 90 minutes.

5. The process according to claim 1 wherein is subsequently included the steps of cooling said substrate and said t-hin film of rhenium thereon, removing the filmcarrying substrate from said vacuum environment, and covering exposed surfaces of the thin film of rheniu'm with an oxidation-preventing protective coating of dielectric material.

6. The process according to claim 5 wherein said dielectric material is silicon dioxide.

7. A process of making highly stable thin film resistors with controlled temperature coefiicient of resistance characteristics, comprising the steps of depositing, in a vacuum environment, vaporized rhenium in a film of predetermined thickness on an electrically insulative substrate heated to a temperature within the range from 400 C. to 700 C., said film thickness being selected to produce the desired resistance value of said film, s aid temperature being selected to produce and to stabilize the desired temperature coefiicient of resistance of said film, and thereafter annealing said substrate and film in said vacuurn environment at a temperature within said range for a period of time sufiicient to stabilize the mechanical and electrical properties thereof.

8. The process according to claim 7 wherein the rheniurn is vaporized by focusing an electron beam on a solid mass of rhenium within said vacuum environment.

9. The process according to claim 7 wherein said substrate is a ceramic material.

10. The process according to claim 7 wherein said substrate is a silica-bearing glass.

11. The process according to claim 7 wherein said period of time is 30 to minutes.

12. The process according to claim 7 wherein is further included the steps of subsequently cooling said substrate and said film of rhenium thereon, removing the filmcarrying substrate from said vacuum environment, and covering exposed surfaces of the film of rheniurn with an oxidation-preventing protective dielectric coating.

13. The process according to claim 7 wherein said dielectric coating is silicon dioxide.

References Cited UNITED STATES PATENTS 2,997,979 8/ 1961 Tassara 117-217 3,015,587 1/1962 MacDonald l17-107.1 1,434,268 10/ 1922 Tillyer 11733.3 2,382,432 8/1945 McManus et al. 117-107 2,616,840 11/1952 Levi 20437 2,904,450 9/ 1959 Island et al. 117-106 2,997,524 8/1961 Ruskin 29---199 3,024,965 3/ 1962 Milleron 204-298 RALPH S. KENDALL, Primary Exmniner.

ALFRED L. LEAVITT, MURRAY KATZ,

Examine/1s. A. GOLIAN, Assistant Examiner. 

1. THE PROCESS OF MAKING HIGHLY STABLE THIN FILM RESISTORS WHICH COMPRISES, IN A VACUUM ENVIRONMENT, THE STEPS OF DEPOSITING VAPORIZED RHENIUM IN A THIN FILM OR AN ELECTRICALLY INSULATING SUBSTRATE HEATED TO A TEMPERATURE WITHIN THE RANGE FROM 400*C. TO 700*C., AND THEREAFTER ANNEALING SAID FILM AT A TEMPERATURE WITHIN SAID RANGE FOR A PERIOD OF TIME SUFFICIENT TO STABILIZE THE ELECTRICAL AND PHYSICAL CHARACTERISTICS OF THIN FILM RESISTOR. 